Conjunction Fallacy

Description

When two events can occur separately or together, the
conjunction, where they overlap, cannot be more likely than the likelihood of
either of the two individual events. However, people forget this and ascribe a
higher likelihood to combination events, erroneously associating quantity of
events with quantity of probability.

Research

Kahneman and Tversky offered the following problem to a
number of people:

Linda is 31, single, outspoken and very bright. She majored
in philosophy. As a student, she was deeply concerned with issues of
discrimination and social justice and also participated in antinuclear
demonstrations.

They then asked whether she was more likely to be (a) a bank teller, or (b) a bank teller
and active in the feminist movement. 86% answered (b).

Example

If someone says an out-of-stock product in a shop may be in within the next
two weeks and it may be in this week, and it may be in tomorrow, then it seems
more likely it will be in sooner rather than later.

So what?

Using it

When persuading about something that is not guaranteed every time, show how
it appears in several different scenarios.

Defending

Remember your mathematics! Just because something can happen in different
circumstances it does not make it more likely.